New Heathkit AS101 owner - First post.

[voice of the theater]

Thanks! I'm in deep!

The phono preamp looks great! I'm glad to hear that you've got your Altecs/stereo system sounding phenomenal.

Of course, many audiophiles prefer bi-amped systems, but some still swear by their passive crossovers. For example, my Altec Model 19's (and Model 14's) have crossovers that have h.f. compensation etc. built in. The passive crossover in these systems is very much an important part of the Model 19 and Model 14 sound and many users of these (and other similar) speaker systems are still using the stock passive crossovers to achieve (what they perceive to be) the "best" results. That Heathkit AS 101 crossover is an excellent crossover--it achieves a very smooth transition from woofer to h.f. driver and I prefer it over several (stock) Altec passive crossovers that I've used in the past. It doesn't have any h.f. compensation etc. so even though I never tried my AS 101's without it, I don't consider it to necessarily be an indispensable part of the AS-101 sound. If I was going to bet, I think you will be able to get better results (possibly by a wide margin) in that system with an active crossover/biamp setup--but it will of course take some work/tinkering to "beat out" the sound of that great AS-101 passive crossover. I'll be very interested in hearing if you end up preferring the bi-amped (Behringer) setup over your Heathkit "stock" crossover.

[alancohen]

Yeah, I'm quite pleased with the crossover but I've heard some amazing things about Bruce's 300B OTL amps that I want to try them. But they only push 1W and while the bass response is supposed to be impressive, I think I'll need a little more slam. Hence the 15W push/pull OTL for the bottom and the 300Bs for the mids and highs. 1W into those 806As should be sublime, especially since I'm bypassing a passive crossover which does drain some power.

I think it will be fun not only playing with the crossover point, but also the slope, volume attenuation for each driver and if I'm not mistaken I can also vary the delay with the DCX2496. That should give me a very high level of control. I will also be able to add another amp to drive some tweeters if I desire, but that's way down the road. I have to hear what these OTL are all about first, especially the 300Bs. The 15W P/P OTL will be in my hands early next week. It will be the perfect Thanksgiving "while watching football" project.

I'll keep you posted....

Oh, and a bonus...the DCX2496 fits in my Ikea Lack rack! I thought is might be too wide. It's like it was made for it.

http://www.flickr.com/photos/alancoh...3033/lightbox/

[Alien_Shore]

Oh, and a bonus...the DCX2496 fits in my Ikea Lack rack! I thought is might be too wide. It's like it was made for it.

Man, I'm glad you posted that. I've been looking for something just like that! Thanks!

[voice of the theater]

......I think it will be fun not only playing with the crossover point, but also the slope, volume attenuation for each driver and if I'm not mistaken I can also vary the delay with the DCX2496. That should give me a very high level of control......

Yeah, I'm sure it will be a fun project and I'll still bet that you'll be able to get better results (possibly by a wide margin) in that system with an active crossover/biamp setup. The timing is right IMHO because you've had some time to acclimate to the passive crossover, so now you've got a good frame of reference/benchmark--now you've heard them for a while in their "stock" state and know what you're trying to beat. Since you're fairly new to Altec I thought I'd throw in my two cents. You probably know that's an 811b horn in the AS 101's. Just wanted to make sure you're aware that their rated frequency response goes down to 800 hz so of course no big deal when tinkering to go lower than that but when you are fine tuning and looking for the ultimate crossover point I wouldn't recommend anything below 800 hz. (your current crossover point is 800 hz). As far as the 416 woofers, most Altec users would agree that a crossover point above 1200 hz is undesirable for them--1200 is pushing them to the limit as far as accurate sound reproduction--lower than that is better for the 416's in many systems IMHO. What sounds best to you is all that matters, but if I had to wager a guess, I'd say the ideal crossover point for your AS 101's is much closer to 800 hz than 1200 hz. Of course, that's just a guess and the fun is in the experimentation so YMMV......

[alancohen]

My goal with the active crossover is not to raise the XO frequency (although I will certainly try it...why not), since my ideal setup would likely be around 300Hz. Obviously not with this horn. It's more to gain the flexibility of two amplifiers and allowing them to feed directly into their respective drivers without passing through inductors and capacitors. I'm building some pretty open, detailed and lightning fast amplifiers and I would like to try and avoid the bottleneck of a passive crossover.

But that brings up a question. I can understand why there would be frequency limits on a driver...it just physically can't vibrate well that fast or slow. But I can't quite get my brain around why the horn itself would have such limitations. I gather the curve of the horn structure is tuned to maximize a certain frequency range, but what would happen if you put a 500Hz signal into an 811b horn, assuming the driver itself could handle it? Of course a sound would come out, but what would be happening to it to make it undesirable? Inquiring minds want to know.

[voice of the theater]

But I can't quite get my brain around why the horn itself would have such limitations. I gather the curve of the horn structure is tuned to maximize a certain frequency range, but what would happen if you put a 500Hz signal into an 811b horn, assuming the driver itself could handle it? Of course a sound would come out, but what would be happening to it to make it undesirable? Inquiring minds want to know.

I think part of the reason that you never got an answer to your question (above) is that it was "buried" at the very end of your post. Now, due to inactivity, your thread is buried under many active threads. This post will bring it back to the "top of the deck".

I'm not sure of the specifics and I know there are people on our forum who would have a much more precise/accurate answer (I've been waiting to reply, hoping that one of these people would chime in with an answer for you). My best attempt at an answer (partly an educated guess) is that it's not just the curve of the horn structure (as you suggested) that determines it's frequency response characteristics, it's also the size. For example, your 811b horn is considerably smaller than Altec's 511b horn which I've owned in the past (which has a rated frequency response down to 500 hz). On the other hand, the smaller Mantaray horn in my Model 14's isn't considered usable below 1200 hz. I think even in the absence of all the scientific data, you will have trouble "picturing" a 500 hz signal coming out of a tiny three or four inch horn (to use an extreme example to illustrate my point). As far as the other part of your question, if I were to venture an educated guess, I would say that if you use a 500 hz crossover through an 811b horn, you will not get a flat response down to 500 hz and there will most likely be a "hole" in your sound (weaker/less output as you approach 500 hz). I don't (and Altec doesn't) recommend going below an 800 hz crossover point with this horn. Not sure what else the 811b horn will "do to" the sound below 800 hz, but I'm (fairly) certain that using a 500 hz crossover with an 811b horn will (hopefully) not blast a hole in the fabric of the space/time continuum.....

Anyone else with more knowledge on this question care to chime in?

[Alien_Shore]

Alan,
I have many of the same questions as you (and others) are recently posing here.
In my research, I've found that the topic is very complex and there aren't many good articles published for the layperson. Take a look at this thread where I've linked some of the better articles that I've found. It will provide a start at least in understanding some of the fundamentals of horns and the science behind them.

[alancohen]

Alan,
I have many of the same questions as you (and others) are recently posing here.
In my research, I've found that the topic is very complex and there aren't many good articles published for the layperson. Take a look at this thread where I've linked some of the better articles that I've found. It will provide a start at least in understanding some of the fundamentals of horns and the science behind them.

That's some pretty heady stuff there. Honestly, I don't think I even want to understand it all. At my age there is only so much room left on the old hard drive. If I let too much more in there I might lose vital data...like my name.

This is an example of what leads to my confusion. Here is a data sheet on the 1225A HF cabinet. On the first page it says the 805B horn "operates from 800Hz to beyond audibility". http://altecpro.com/pdfs/vintage/Spe...%20Systems.pdf

Then down below it says the 1225A enclosure, with only the 805B horn in it, has a frequency range from 300Hz to 8000Hz. That makes no sense to me.

I'm now in the camp of just try it and see what happens. I know my 806A CDs go down to 500Hz. I'm going to tune my crossover that low and see what I hear. If I like it, who cares, right? If not, I'll adjust it until I do. The good thing is I know what sounds good...to me at least. I have a sound in my head that I'm trying to achieve. I really don't care what the numbers are.

[gearfreak]

...gather the curve of the horn structure is tuned to maximize a certain frequency range, but what would happen if you put a 500Hz signal into an 811b horn, assuming the driver itself could handle it? Of course a sound would come out, but what would be happening to it to make it undesirable? Inquiring minds want to know.

I may have missed the original post about undesireability... so maybe this is not entirely on target.
I think the overall point is that the horn will not load the driver below Fc. This creates an impedance mismatch. So you loose efficiency, perhaps to the point of no sound coming out.

Take the analogy to the next step and think of it this way.

If you strapped a 15" woofer to an 811, would sound come out? Yes.
Would 30hz come out? Probably: 'not really'.

[alancohen]

Actually, GF, your post is exactly on target.

I guess the first thing is I don't get how a horn by itself, basically an inert flared tube, has an "impedance" at all. I understand the impedance of AC current in a circuit, but how does this apply to a horn? I assume it has something to do with sound waves moving in an air medium through a restricted space.

I gather that the lower the frequency, the longer the sound wave so it makes sense to me that the length of a horn could influence frequency efficiency, but length seems to be irrelevant in that shorter horns can still be rated lower than longer horns of a difference shape. I don't get it.

The good news is at this point I don't have to. I have what I have as far as equipment and you gotta dance with who you brung. I'm just trying to figure out what the ramifications are of trying to put 500Hz through an 811B horn. I'm going to try it anyway and see what I hear. It will either sound better to me or worse. It's a luxury to know why, not a necessity.